The present invention relates to magnetic flow meters that sense liquids flowing in industrial process plants. In particular, the present invention relates to noise detected by electrode circuits in such magnetic flow meters.
Magnetic flow meters utilize an insulated flowtube that carries liquid flowing past an electromagnet and electrodes. The electrodes are sealed in the flowtube and make contact with the flowing liquid. The electrodes sense an electromotive force (EMF) magnetically induced in the liquid, and proportional to flow rate according to Faraday""s law of electromagnetic induction.
Along with this flow EMF, undesired noise is often received from the electrodes of a magnetic flow tube. This electrical noise is indicative of undesired operating conditions associated with the flowing liquid such as poor liquid grounding connections, excessive electrical resistivity in the flowing liquid, particles in the liquid impacting the electrodes, ongoing dissolving of particles in the liquid, ongoing chemical reactions in the liquid, entrained gas bubbles and the like.
This xe2x80x9celectrode noise,xe2x80x9d that originates in the liquid, can give rise to measurement instability or variability in the flow output of the transmitter that can go undiagnosed by the operator of the process plant for long periods of time.
A magnetic flow transmitter with noise diagnostics and a method of noise diagnostics are disclosed. The transmitter includes a digital processor that generates spectral components of a digitized, amplified electrode signal from a flow tube carrying a flow of a liquid. These spectral components extend over a fundamental frequency that is selected for a coil driver output provided to the flow tube and also extend over a frequency of at least one known potential noise source. The digital processor generates a diagnostic output as a function of at least one spectral component at or near the frequency of the noise source. The digital processor also generates a flow output as a function of at least one spectral component at or near the fundamental.
The diagnostic output indicates the frequency and amplitude of noise detected from the fluid and whether the noise is large enough to affect the stability of the flow output so that corrective action can be taken if needed.